The invention relates to a method of manufacturing polycrystalline diamond layers, in which diamond crystallites are deposited by means of Chemical Vapour Deposition (CVD) on a substrate heated to a temperature ranging between 450.degree. and 1200.degree. C. from a gas phase comprising hydrogen and .ltoreq.30% of a carbon-containing gas at a pressure ranging between 10.sup.-5 and 1 bar.
It is known that the manufacture of synthetic diamond is not only possible at a high pressure and a high temperature but also at atmospheric pressure or in the sub-pressure range by way of the chemical gas phase deposition process (CVD) (in this respect reference is made to U.S. Pat. No. 3,030,187).
In addition to the deposition parameters of pressure, concentration of the reactants in the gase phase and temperature of the gas phase, the condition of the substrate surface also plays a role in the morphology of layers deposited from the gas phase by means of CVD and the period of time required to build up the layer. It is, for example, advantageous to roughen substrate surfaces by means of grained grinding means of or with particularly hard materials, or by spraying powders, particularly of hard materials, on the substrate surface. Both methods of preparing the substrate surfaces have, however, certain drawbacks: the substrates must always be prepared outside the reactor used for depositing the diamond layers, which always involves the risk of contamination of the substrate surfaces, particularly because of their relatively large surface roughness. Due to spraying hard material powders, for example, by applying diamond powders manufactured by means of high-pressure synthesis, a homogeneous growth of the layer thickness is not only jeopardized by the introduction of contaminations but also because it is difficult to apply powder layers on the substrate in a homogeneous manner. The powder material which is applied may be easily lost when the substrates are introduced into the reactor or it may be displaced in its surface regions, which in both cases leads to disturbances of the subsequent build-up of layers. To mitigate these unwanted effects, it has been attempted to nucleate the substrate surfaces by applying diamond powder-containing suspensions. It is true that the homogeneity of the substrate surface nucleation is then improved, but an additional risk of contamination of the substrate surfaces is involved during the steps of suspension and removal of excess liquid phase. In all cases it is a drawback to resort to diamond powders manufactured by means of high-pressure synthesis for nucleating substrate surfaces, because these powders may generally be contaminated during manufacture with impurity atoms such as, for example iron, nickel, titanium, which impurities are subjected, along with the nucleated substrates, to the deposition process for in situ formation of diamond layers.
In the in situ manufacture of diamond layers a further problem is that it is desirable to nucleate the substrate surfaces several times during the process of growing the layer: for this purpose the process of growing the layer must be interrupted each time, which involves the risk of contaminating the diamond layer which has already been deposited, because the substrate must be removed from the reactor whenever a new nucleation step is started.